Image scanners convert a visible image on a document or photograph, or an image in a transparent medium, into an electronic form suitable for copying, storing or processing by a computer. An image scanner may be a separate device or an image scanner may be a part of a copier, part of a facsimile machine, or part of a multipurpose device. Reflective image scanners typically have a controlled source of light, and light is reflected off the surface of a document, through an optics system, and onto an array of photosensitive devices. The photosensitive devices convert received light intensity into an electronic signal. Transparency image scanners pass light through a transparent image, for example a photographic positive slide, through an optics system, and then onto an array of photosensitive devices.
Photosensor arrays typically have thousands of individual photosensitive elements. Each photosensitive element, in conjunction with the scanner optics system, measures light intensity from an effective area on the document defining a picture element (pixel) on the image being scanned. Optical sampling rate is often expressed as pixels per inch (or mm) as measured on the document (or object, or transparency) being scanned.
Photosensor arrays for image scanners commonly have three or four rows of sensors, with each row receiving a different band of wavelengths of light, for example, red, green and blue. Each row may be filtered, or white light may be separated into different bands of wavelengths by a beam splitter.
Sensor arrays are typically exposed for a fixed amount of time, and electric charges for each sensor element are transferred to one or more charge shift registers. The charges are then serially shifted bucket-brigade style in the charge shift registers to amplifiers and analog-to-digital (A/D) converters.
Bit depth is the number of bits captured per pixel. Typically, a pixel is specified in a three-dimensional color space with a fixed number of bits in each dimension. For example, a pixel may be specified in red, green, blue (RGB) color space, with 8 bits of red information, 8 bits of green information, and 8 bits of blue information, for a total of 24 bits per pixel. Alternatively, a pixel may be specified in a cylindrical color space in which the dimensions are luminance, chrominance, and saturation. Alternatively, a three-dimensional CIE color space may be used, for example, CIELAB or CIELUV, where one dimension is luminance.
During exposure to light, the primary noise source (called shot noise) is related to conversion of photons to electrons, and the noise increases with the square root of the signal. Even if a sensor is receiving no light, some thermal noise (called dark noise) may occur. Thermal noise (dark noise) increases with time, temperature, and photosensor area.
The sensitivity of the human visual system to light intensity is approximately logarithmic. That is, the human visual system is very sensitive to intensity changes in dark areas and shadow areas and the sensitivity to intensity change decreases with increasing intensity. Therefore, for a fixed level of noise, the noise is visually more apparent in the dark areas of an image. Photosensor signals are smaller for dark areas of an image, so that the signal-to-noise ratio becomes a particular concern for the dark areas of an image. Areas of an image with slowly varying color, particularly dark colors, require accurate lower order bits of bit depth, and high signal-to-noise, to accurately reproduce the smooth tone and texture of the original. Accordingly, because of the sensitivity of the human visual system in dark areas, and because of the signal-to-noise ratio in dark areas, and because of the need for accurate lower order bits in areas with slowly varying color, there is a need to reduce thermal noise.
Typically, for each generation of products, optical sampling rate increases, requiring smaller sensor sizes. Increased sampling rate also results in the sensors gathering less light, so that if a specified signal-to-noise ratio is required, then other changes may need to be made, such as increasing the light intensity, making the lens in the optical system larger, or increasing exposure times, each of which affects cost or performance. There is a need for a scanner that provides accurate lower order bits in bit depth, and high signal-to-noise, with a high optical sampling rate, with minimal impact on cost and performance.